1. Technical Field of the Invention
This invention relates to radio telecommunication systems and, more particularly, to a system and method for dynamically increasing the capacity of a code division multiple access (CDMA) radio telecommunications network.
2. Description of Related Art
In radio telecommunications networks, a wide variety of services such as voice, video, data, and image services are available to subscribers. In using these radio telecommunications networks, the most precious resource is the radio spectrum. To maximize the effectiveness of the networks, code division multiple access (CDMA) has been developed to facilitate communications involving a large number of system users. There are other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA), however, the spread spectrum modulation technique of CDMA has proven to offer many advantages over the other methods.
In a CDMA system, a unique binary spreading sequence, known as a code, is assigned for each call to each user. Multiplied by the assigned code, the user's signal is spread onto a channel bandwidth wider than the user signal bandwidth. The ratio of the system channel bandwidth to the user's bandwidth is known as the "spreading gain." All active users share the same system channel bandwidth frequency spectrum at the same time. The signal of each user is separated from the other users' signals at the receiver by issuing a correlator keyed with the associated code sequence to "de-spread" the desired signal. In this manner, an increased number of simultaneous users can be served.
In addition to serving an increased number of users, the CDMA system uses a technique known as "macro-diversity" to improve the quality of the radio reception in the radio telecommunications network. The macro-diversity mode involves the use of two or more simultaneous links from two or more cells or base stations to a single mobile station in order to improve the quality or performance of the reception of the radio signal. The mobile station and base station receivers both employ a number of correlators. Receivers using parallel correlators, which are sometimes called rake receivers, allow individual path arrivals to be tracked independently, and the sum of their received signal strengths is then used to demodulate the signal. While there may be fading on each path arrival, the fading on each channel is independent of the other. Thus, demodulation based on the sum of the signals is much more reliable.
However, there are disadvantages in the existing system. By employing the macro-diversity mode, more resources of the base station are utilized. In addition, a high percentage of calls take place in the macro-diversity mode. Therefore, by the use of the macro-diversity technique, there is a greater likelihood of overloading the radio telecommunications network. Although the CDMA system using the macro-diversity mode provides for a clearer reception of radio telecommunication signals, a large amount of network resources is utilized. There may arise a situation where services are needed for additional users in a fully loaded system, and where the high quality provided by the macro-diversity mode may not be necessary to maintain. At present, there is no method for dynamically increasing the capacity of a fully loaded macro-diversity radio telecommunication network.
Although there are no known prior art teachings of a solution to the aforementioned deficiency and shortcoming such as that disclosed herein, prior art references that discusses subject matter that bears some relation to matters discussed herein are U.S. Pat. No. 5,109,390 to Gilhousen et al. (Gilhousen '390), U.S. Pat. No. 5,265,119 to Gilhousen et al. (Gilhousen '119), and International Publication Number WO 95/35637 (Publication '637).
Gilhousen '390 discloses a spread spectrum receiver subsystem in a CDMA cellular telephone having a searcher receiver for scanning the time domain so as to use processing gain and time discrimination properties of spread spectrum coding to determine the location in the time domain and the received signal strength of multiple receptions of a pilot signal traveling upon one or more physical propagation paths to reception. The searcher receiver provides a control signal for acquiring and demodulating a spread spectrum communication signal. Thus, Gilhousen '390 describes a CDMA receiver with a flexible capability in the time domain. However, Gilhousen '390 does not teach or suggest a method or system for increasing the capacity of a CDMA system operating in the macro-diversity mode.
Gilhousen '119 discloses a power control system for a cellular mobile telephone system in which system users communicate information signals between one another via at least one cell site using CDMA communication signals. The power control system controls transmission signal power for each cellular mobile telephone in which each cellular mobile telephone has an antenna, transmitter and receiver and each cell-site has an antenna, transmitter and receiver. The signal power transmitted from a cell-site is measured at the mobile unit. Transmitter power is adjusted at the mobile unit by decreasing or increasing the received signal power in a manner opposite to the transmitter's power strength. A command signal is generated at the cellsite and transmitted to the mobile unit for further adjusting of the mobile unit transmitter's power corresponding to deviations in the cell site's received signal power. The feedback scheme is used to further adjust the mobile unit transmitter power so that the mobile unit's transmitted signals arrive at the cell-site at a desired power level. In a cell diversity situation, the mobile unit's transmitter power is adjusted to prevent unnecessary increases in mobile unit transmitter power level. Thus, Gilhousen '119 describes a method and apparatus for controlling transmission power in a CDMA cellular mobile telephone system. However, Gilhousen '119 does not teach or suggest a system or method for increasing the capacity of a loaded CDMA system in the macro-diversity mode.
Publication '637 discloses a method in a CDMA cellular radio network having in each cell at least one base station, which communicates on a specified traffic channel, with mobile stations in its area. The signal of the users is divided into bursts in the time domain. To achieve high traffic capacity along with high-quality connections, the base station monitors the load state of the traffic channel, and transmits the information about the load state of the traffic channel to the mobile stations. The number of bursts transmitted between the mobile stations and the base stations per time unit is controlled on the basis of the delay state of the bursts and the load state information computed at the base. Thus, Publication '637 describes a method of regulating the communications load on each channel in a CDMA network using a burst control signal. However, Publication '637 does not teach or suggest a system or method to increase the capacity of a CDMA system by dynamically changing the parameters affecting the macro-diversity mode.
A method and system is needed that dynamically increases the capacity of a CDMA system in the macro-diversity mode, without disrupting any ongoing calls. It would be a distinct advantage to have a system and method for accommodating additional mobile stations in a CDMA radio telecommunications network. It is an object of the present invention to provide such a system and method.